The present invention is concerned With a vapor deposition apparatus. As known in the art, such apparatus is used for depositing a film from vapor onto a substrate. For example, the substrate may be a wafer, such as a silicon wafer, and a film of deposited vapor on that substrate is used in the manufacture of semiconductors and other like devices. Alternatively, such an apparatus can be used for causing the uniform evaporation of a film from a substrate, such as a wafer, and this can be used in an etching process by choosing the chemical reaction between the reactants and the film.
In such an apparatus, it is necessary to have an enclosure such that the vapor flows through the enclosure and contacts a very hot susceptor disposed therein so that the desired deposition or evaporation may take place. The temperature of the susceptor is set for particular vapors and wafers and can vary according to those parameters, which are well known in the art. While the specific arrangement of such an enclosure can vary considerably, these enclosures, as major components, include a first end wall with a vapor inlet therein, a second end wall with a vapor outlet therein, side walls, a top wall, a bottom wall, a susceptor having a top surface, gas flow control means for causing a flow of the vapor from the inlet to the outlet, and heating means for heating the susceptor to high temperatures required for specific vapors/wafers. The present apparatus has those same major components.
There are two main types of vapor deposition apparatus. The first is the so-called "horizontal" type, and the second is the so-called "vertical" type. In the horizontal type apparatus, the susceptor is disposed horizontally within the enclosure and the vapor flows parallel to the susceptor. In the vertical type, the susceptor is also disposed horizontally, but the vapor flows transverse thereto, e.g. from a top portion with an inlet, along the side walls, to and along the susceptor, and out an outlet on a bottom portion.
In both of these types of apparatus, the vapor flow through the enclosure may be entirely the chemical compound which is intended to be deposited, or it may be a combination of the vapor of that compound and a carrier gas. Generically, whether the flow is entirely of the chemical compound or mixed with a carrier gas, the flow is referred to as the "gas flow", and that term will be used hereinafter.
However, these conventional devices have a substantial disadvantage in that the very hot susceptor, placed on the bottom wall, of, for example, a "horizontal" type apparatus, necessarily causes thermal convection flows of the gas within the enclosures. These convection flows, coupled with the main gas flow through in the enclosure, causes stable gas rolls in the flow of the gas, such that the temperature near the susceptor or, for example, a wafer thereon, is never horizontally uniform, even though the temperature of the susceptor itself is uniform. This vertical temperature non-uniformity may be referred to as a vertical temperature gradient, i.e. a gradient extending vertically from the susceptor or wafer thereon. Since the mass flux of the vapor onto the wafer is strongly dependent on the temperature gradient vertical to the susceptor or wafer, the growth of a deposited film, therefore, becomes non-uniform.
In order to achieve a more uniform growth of the film, the prior art has considered it necessary to exercise some control of the gas flow, and various prior art approaches have been suggested for this purpose. One suggested method is that of rotating the susceptor so as to smooth out the local temperatures near the susceptor or wafer, i.e the vertical temperature gradient, and, hence, smooth out deposition rates across the susceptor, but this approach considerably complicates the apparatus. Another suggested method is that of reducing the pressure inside the enclosure, but this even further complicates the apparatus and involves vacuum pumping systems.
A recent and important suggestion is that of suppressing the stable gas rolls by controlling the temperature of the side walls of the enclosure (see laid open Japanese Application No. 137620/1989). This approach uses special heaters along the side walls. As a result, the gas flow becomes unsteady, and hence no stable gas rolls are possible. When the so-called "horizontal" apparatus is so controlled, a time-average horizontal uniformity of the vertical temperature gradient of gas at the susceptor is achieved, however, only when the vapor flow is within certain Reynolds numbers (Re) and Grashof numbers (Gr). Further, this approach can achieve uniform growth of the film only in the direction of the susceptor which is transverse to the direction of the gas flow and cannot achieve uniform film growth in the direction of the gas flow. This is because the concentrations of the vapor to be deposited from the gas flow decrease as the gas flow proceeds through the enclosure, since the deposited vapor depletes the vapor content in the gas flow.
In order to improve this situation, it has been suggested to reduce the cross-sectional area of the enclosure near a downstream portion thereof so as to increase the flow speed of the gas. With this, the concentration of the vapor per unit volume can be increased. However, the optimum geometrical shape of such an enclosure, for this purpose, is not simple, and the apparatus becomes very complicated.